Recently, a hydrophilic porous fluoropolymer membrane was disclosed in U.S. Pat. No. 5,130,024. In this patent, normally hydrophobic fluoropolymer membrane is rendered hydrophilic by coating the pores with a hydrophilic fluorine-containing copolymer. Increasing the hydrophilicity of filtration membranes increases their efficiency in filtration applications involving filtering aqueous compositions. A typical such copolymer taught by this patent is a copolymer of a) a monomer of the formula CXY.dbd.CFZ where Z can be fluorine or hydrogen, and X and Y can be H, F, Cl or CF.sub.3 (preferably they are all F), and b) a monomer of the formula ##STR1##
This copolymer, after saponification of the acetate group to hydroxyl, is coated on the pores of the membrane to provide hydrophilicity to the membrane. The coating is durable because of the fluorocarbon attraction between the membrane CF.sub.2 groups and the copolymer CF bonds. Hydrophilicity is provided by the conversion of the ##STR2## (acetate) side groups in the copolymer to --OH groups, thus forming vinyl alcohol recurring units in the copolymer chain. This copolymer will be referred to hereinafter sometimes as the VOH copolymer.
The VOH copolymer coating of hydrophobic substrates increases their surface free energy significantly. This makes such coated substrates spontaneously wettable by high surface tension liquids such as water, which consequently opens up the use of such substrates to aqueous filtration applications. Some other potential applications are in the area of increased adhesion to high surface energy substrates, and use in biomedical devices, among others.
While the potential for such applications are promising, the VOH-copolymer coating suffers from two drawbacks: (1) Loss of hydrophilicity because of lack of heat resistance at temperatures above 120.degree. C.; (2) Lack of resistance to organic solvents such as methyl alcohol and N, N-dimethylformamide.
When a VOH-copolymer coated membrane is heated at 120.degree. C. and above, it has been observed that the water-wettability of the substrate becomes reduced, i.e., it is either no longer water-wettable or it requires a longer contact time with water or a higher contact pressure in order to become fully wet. Steam sterilization at 120.degree. C. for 0.5 hour also has such a deleterious effect on the water-wettability of the VOH-copolymer coated substrates. Analysis via nuclear magnetic resonance spectroscopy as well as infrared spectroscopy of the VOH-copolymer before and after heating demonstrates that the loss of water-wettability upon heating is not caused by a chemical change in the VOH-copolymer. This leaves the possibility that the deleterious effect on water-wettability is caused by a physical change of the VOH-copolymer. This change could be in the form of C--OH bond rotations. If the hydroxy groups are rotated away from the surface of the substrate into its bulk, the hydroxy groups will no longer be in the optimum orientation for "receiving" and hydrogen bonding with incoming water.